The psycho acoustic model in an audio encoder, e.g. in a MPEG 1 or MPEG 2 audio system, calculates a masking threshold for data reduction purposes. The masking threshold may vary for each subband of the audio signal. The so called bit allocation process defines for each subband due to the determined masking threshold, with which resolution the subband samples should be quantized. This will vary in time with each block of audio signal samples. Therefore the necessary bitrate for an optimal sound quality should also vary for the different blocks. In practice in an MPEG1 audio system, the available desired bitrate is normally fixed for each audio channel. Because, with respect to the desired bitrate, the exact number of bits for the various parameters and data required for transmission cannot be estimated exactly in advance some bits are left over or some bits are missing. Usually a simple allocation strategy is used to overcome this problem, e.g. by assigning a nearly equal number of such bits for each subband.
The present allocation strategies mentioned above do not take into account the psycho acoustic model used in the encoder. Nowadays, encoders are optimised for a small range of bitrates. In spite of this fact such audio coders are used also for operation in a larger range of bit rates, but they are not optimised for doing so. The different bitrates may occur between the left and right channel and/or additional channels (left surround, right surround, center) and/or in MPEG LayerIII where the desired bit rate can vary from frame to frame.